Spheroidal graphite cast iron is in wide use as components for vehicles including motor vehicles, machine parts, etc., because the spheroidal graphite cast iron has excellent tensile strength and ductility. In particular, this spheroidal graphite cast iron is used in brake calipers, which are important as safety components for vehicles such as motor vehicles, in order to ensure the quality thereof.
Since there recently is a desire for weight reduction and size reduction in these components, the spheroidal graphite cast iron members to be used are also required to be reduced in thickness. In the case where a spheroidal graphite cast iron member is produced so as to be reduced in thickness, a cooling rate is increased in the thin-wall part thereof and this results in the formation of a chill phase (abnormal structure). Since this chill phase has an exceedingly hard structure, the machinability of the spheroidal graphite cast iron member is undesirably reduced.
Because of this, spheroidal graphite cast iron members having a thin-wall part, in particular, components for motor vehicles, are frequently required to be inhibited from having a chill structure and to retain a high level of balance between tensile strength and ductility. Consequently, when a spheroidal graphite cast iron member is produced, the cast molten iron is subjected to a spheroidization treatment and further subjected to an inoculation treatment multiple times. In the spheroidization treatment, a spheroidizing agent containing a rare-earth element (rare earth) is generally used in order to more reliably conduct spheroidization and graphitization.
For example, Patent Documents 1 to 4 disclose spheroidizing agents containing rare earth in a given amount (in the range of about 0.5 to 9% by mass) and spheroidal graphite cast iron produced using the spheroidizing agents. Rare earth not only has the effect of accelerating spheroidization graphitization on the basis of both a deoxidizing and desulfurizing function and the function of lowering the action of spheroidization-inhibitory elements but also serves, for example, to accelerate graphitization, prevent chill phase formation, inhibit chunky graphite formation, and inhibit fading, on the basis of the effect of yielding graphite nuclei, etc. Hence, rare earth is element exceedingly profitable for spheroidal graphite cast iron. Especially in the production of spheroidal graphite cast iron for use in components for motor vehicles, use of a spheroidizing agent containing such rare earth is regarded as essential for preventing chill phase formation in the thin-wall part.